1. Field of the Invention
This invention relates generally to an objective lens assembly particularly suited for use in an optical disc player in which a light beam is directed against a disc or other record medium surface for optically reading information recorded in a track on such surface.
2. Description of the Prior Art
It has been proposed, for example, in U.S. patent application Ser. No. 06/336,551, filed Jan. 4, 1982, and having a common assignee herewith, to provide an objective lens assembly for an optical disc player in which an objective lens is eccentrically mounted in an annular lens mount from which a cylindrical body depends, and the cylindrical body is slidable within a cylindrical bearing for both axial and rotary movements relative to the latter. The cylindrical bearing is secured within an inner cylindrical yoke to which an outer cylindrical yoke is connected, at the bottom, by an annular yoke connection, and an annular magnet is interposed in the outer yoke to complete a magnetic circuit in which magnetic flux extends across an upwardly opening annular space or gap defined between the inner and outer yokes. The outer annular yoke has a pair of diametrically opposed cutouts therein so that the magnetic flux or field between the inner and outer yokes will be very substantially reduced in the regions of such cutouts. A flange extends radially outward from the periphery of the lens mount, and a cylindrical bobbin depends axially from the outer periphery of such flange into the annular gap between the inner and outer yokes. A focusing coil is wound circumferentially around the outer surface of the cylindrical bobbin so that a driving current flowing in the focusing coil can coact with the magnetic flux extending across the gap for causing axial displacement of the objective lens parallel to the central axis of the cylindrical body and bearing. Also mounted on the cylindrical bobbin are substantially rectangular tracking coils having first and second portions extending substantially parallel to the axis of the cylindrical body and bearing and through which a driving current supplied to the tracking coils will flow in opposed directions, with the first portions of the tracking coils being located between the inner and outer yokes for interacting with the magnetic flux and thereby effecting desired rotary movements in response to the driving current, while the second portions of the tracking coils are located proximate to the cutouts in the outer yoke for reducing the electromagnetically induced force tending to oppose such desired rotary movements.
When the above-described lens assembly is in use, a light beam, in passing through the objective lens, extends longitudinally through the cylindrical body and a focusing error signal may be applied to the focusing coil for effecting axial movements of the cylindrical body within the bearing to cause corresponding movements of the objective lens for correcting focusing errors. Further, in response to a tracking error signal applied to the tracking coils, the resulting driving current flowing through the tracking coils interacts with the magnetic flux for turning the cylindrical body relative to the bearing, and the eccentricity of the objective lens relative to the cylindrical body may be in a direction to cause displacement of the optical axis of the objective lens in the radial direction of the record disc being scanned for correcting a tracking error.
However, it has been found that the abovedescribed objective lens assembly has less sensitivity than may be desired, particularly in respect to the correction of tracking errors, and that the arrangement of such lens assembly inherently limits the increases in such sensitivity that may be achieved therewith. The sensitivity of the assembly for correcting tracking errors can be increased by: (1) Increasing the eccentricity of the objective lens, that is, the radial distance between the optical axis of such lens and the axis about which the lens mount is turned for correcting tracking errors; (2) Reducing the frictional resistance to turning of the objective lens mount; (3) Increasing the driving torque of the rotating motor constituted by the tracking coils and the magnetic circuit; and (4) Reducing the weight of the structure supporting the objective lens and movable therewith for correcting tracking errors. However, in the case of the previously proposed objective lens assembly described above, since the path of the light beam through the objective lens extends axially within the cylindrical body which is rotatable within the cylindrical bearing, any increase in the eccentricity of the objective lens relative to the axis of rotary movement requires a corresponding increase in the diameters of the cylindrical body and bearing, as a result of which there is a corresponding increase in the frictional resistance to the rotary movements for correcting tracking errors and also an increase in the weight of the supporting structure movable with the objective lens. Such increases in frictional resistance to the rotary movements and in the weight of the structure that needs to be moved more than overcome the advantageous increase in eccentricity and any increase in the driving torque of the rotating motor achieved by increasing the diameter of the bobbin on which the tracking coils are mounted. Therefore, substantial improvements in the sensitivity of the lens assembly for correcting tracking errors are not to be expected from increasing the eccentricity of the lens.
Furthermore, since the cylindrical bearing which guides the cylindrical body supporting the lens mount is secured to the inner surface of the inner yoke, a leakage flux is generated toward the cylindrical body and bearing from the flanks of the outer yoke defining the cutouts in the latter. Such leakage flux reduces the density of the remaining magnetic flux available to coact with the first portions of the tracking coils for causing the desired rotary movements to correct a tracking error. Moreover, the leakage flux will undesirably coact with the second portions of the tracking coils situated proximate to cutouts for decreasing the driving torque in the desired direction.